The Marketplace is a store that features apps designed for any device that runs Firefox OS, Firefox for Android or Firefox desktop. It makes finding your favorite apps and discovering new apps easy. What's great is that the apps you download or purchase can be used on multiple devices and platforms.
For more information, please see:
Sign in to the Marketplace to use your apps on multiple devices
You don't have to sign in to the Marketplace to download free apps but if you want to purchase an app, you will be asked to sign in. If you sign in to the Marketplace, all of the apps that you download (free and purchased) will be added to your account. You can then easily access these from another Firefox OS device or from an Android device.
You can sign in to the Marketplace by tapping the menu button in the upper-left of the main screen and then choosing Sign In.
Once you are logged in, you can always access the apps you've downloaded:
Tap the menu button in the upper-left of the main screen and then choose My Apps.